The present invention relates to a current-mode control step-up converter.
Electronic apparatuses, such as portable apparatuses, use a battery as a power supply. The voltage of this battery is stepped up to a desired voltage using a step-up converter and is used as a power supply voltage for various electronic circuits. The step-up converter is a switching DC-DC converter comprising an inductor, a main switch and a rectifier. The current-mode control system being excellent in stability is known as a control system for the converter. FIG. 10 is a circuit diagram showing the current-mode control step-up converter disclosed in FIG. 4 of Japanese Patent Application Laid-open No. 2000-134923.
The conventional step-up converter shown in FIG. 10 comprises an input DC power supply 201 for supplying an input DC voltage Vi; an inductor 202 and a main switch 203 connected in series with each other and further connected in parallel with the input DC power supply 201; a diode 214 serving as a rectifier and connected between the connection point of the main switch 203 and the inductor 202 and the output of the step-up converter; an output capacitor 205 for supplying an output DC voltage Vo to a load 206; a reference voltage supply 207 for outputting a reference voltage Vr; an error amplifier 218, to which the output DC voltage Vo and the reference voltage Vr are input, for outputting an error signal Ve; a current detector 219 for detecting the current of the inductor 202 and outputting a current detection signal Vc; and a drive circuit 210, to which the error signal Ve and the current detection signal Vc are input, for turning ON/OFF the main switch 203.
With the configuration described above, the drive circuit 210 ON/OFF controls the main switch 203 so that the current detection signal Vc follows the error signal Ve. Hence, the current of the inductor 202 is adjusted so that the output DC voltage Vo becomes equal to the reference voltage Vr. Since the inductor equivalently serves as a current supply in the current-mode control system for not only a step-up converter but also other apparatuses, the influence of the LC resonance between the inductor and the output capacitor to the control operation can be suppressed. Therefore, the current-mode control system makes it possible to attain a DC-DC converter being excellent in control stability and response.
Japanese Patent Application Laid-open No. 2000-134923, featuring a current detector and its insertion position, discloses not only the step-up converter shown in FIG. 10 and serving as a current-mode control step-up converter but also a step-up converter in which a current detector is inserted on the output side of an inductor and a step-up converter in which a current detector is inserted in series with a main switch.
The conventional current-mode control step-up converter detects the current of the inductor or the main switch, and controls its peak value, thereby stabilizing the output. Hence, the main switch must be turned ON, and current is required to flow therethrough. Hence, there exists a minimum ON time (minimum duty ratio). The duty ratio is the ratio of the ON time in one switching cycle. During the ordinary operation, as the duty ratio is larger, the output DC voltage Vo is higher. In other words, as the duty ratio is larger, the step-up ratio (Vo/Vi) of the step-up converter is larger. Conversely, as the duty ratio is smaller, the step-up ratio is smaller and approaches one. The existence of the minimum duty ratio in the step-up converter causes a problem of impairing the stability of the output when the input and output voltages are close to each other.
In addition, since the ON state of the main switch tends to be maintained until the detected current reaches its target value, the duty ratio falls into one if no limitation is set. In the circuit configuration of the step-up converter, it is obvious that no power can be output via the rectifier when the duty ratio is one. Furthermore, the step-up ratio has an upper limit owing to the influence of resistance components including the resistances of the main switch and other components during conduction and wiring resistances, and it is necessary to set the maximum duty ratio so that the step-up ratio does not reach this upper limit.